Horn antenna

ABSTRACT

A horn antenna produced by angularly flaring the end of a circular of rectangular waveguide into the shape of a horn to be used for radiating or receiving radio waves directly into or from space or to be used as the feed radiator of an antenna for illuminating a mirror and/or a reflector, in which a band of dielectric material is arranged circumferentially on a part of the inner surface of the horn so as to be symmetrical with the axis of the horn to obtain a field pattern having low side lobes and substantially the beam width with respect to at least two perpendicular planes of polarization. The circumferential band of dielectric material may be replaced by two pairs of opposed plates of dielectric material, which pairs are spaced longitudinally of the horn.

United States Patent [72] lnventors Toshio Sato Sagamlhara-shi; Matsuichi Yamada, Yokohama-shi, both of Japan [21] Appl. No. 873,988 [22] Filed Nov. 4, 1969 [45] Patented Nov. 30, 1971 [73] Assignee Kobusal Denkshin, Denwa Kahushiki Kaisha Tokyo-To, Japan [32] Priorities Nov. 5, 1968 [33] Japan [31] 43/80417;

June 7, 1969, Japan, No. 44/449130 {54] HORN ANTENNA 5 Claims, 8 Drawing Figs. [52] 0.5. CI 343/756, 343/786, 343/911 [51] Int. Cl l-10lq 13/02, l-lOlq 15/08 [50] Field of Search OTHER REFERENCES Phase Correction by Dielectric Slabs in Sectoral Horn Antennas," Quddus and German in IRE Transactions on Antennas and Propagation Vol. AP- 9 No. 4 July 1961 TK7800 12; pages4l3-4l5 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Marvin Nussbaum Attorneys-Robert E. Burns and Emmanuel J. Lobato ABSTRACT: A horn antenna produced by angularly flaring the end of a circular of rectangular waveguide into the shape of a horn to be used for radiating or receiving radio waves directly into or from space or to be used as the feed radiator of an antenna for illuminating a mirror and/or a reflector, in which a band of dielectric material is arranged circumferentially on a part of the inner surface of the horn so as to be symmetrical with the axis of the horn to obtain a field pattern having low side lobes and substantially the beam width with respect to at least two perpendicular planes of polarization. The circumferential band of dielectric material may be replaced by two pairs of opposed plates of dielectric material, which pairs are spaced longitudinally of the horn.

PATENTEU unvao um: SHEET 1 OF 2 24, 55

PHASE ANGLE OF LEAD ANGLE Fig. 3 Fig. 4

PATENTED nuvso I971 SHEET 2 0F 2 3 6 ANGLE HORN ANTENNA This invention relates to conical and pyramidal horn antennas such as may be used as the feed radiator of a parabolic antenna, such as Cassegrain antenna. To radiate or receive radio waves of a desired plane of polarization, conical horn antennas are broadly employed since they have radiation patterns which symmetrical with respect their axes. Moreover, hornreflector antennas are used as independent antennas or as feed radiators of other independent antennas.

In this case, since the electric field pattern for the main mode of a circular waveguide used to excite a conical horn is not symmetrical with respect to the axis of the circular waveguide, the radiation pattern of the conical horn has substantial defects. One of these defects is the fact that the field pattern with respect to a plane (i.e.; E-plane) including the axis of the horn and the-electric field is not identical with a field pattern with respect to a plane (i.e.; I l-plane) perpendicular to the E-plane. In other words, the field pattern of the E-plane has relatively high side lobes and a beam width which is narrower by to 30 percent than the beam width for the H- plane field pattern. Therefore, if the conical horn is employed for illuminating the mirror of a Cassegrain antenna, the efficiency of this illumination is relatively low because of nonuniform characteristic of illumination. Moreover, since a part of the power radiated from the horn leaks from the main reflector of the antenna, the noisetemperature of the antenna rises due to noise pickup from ground of the leaked components. Another of the above-mentioned defects is the fact that the efficiency of the antenna is relatively low since the center of the phase pattern of the conical horn cannot be arranged to correspond to the focus of the Cassegrain antenna, since the conical horn assumes distinct phase patterns with respect to the E-plane and the H-plane, respectively, and since the center points on the two phase patterns are also different from one another.

To eliminate the above-mentioned defects, some improvements are proposed in the conventional art. One of these improvements is a metal step provided at the connection part between the circular wave guide and the horn. In accordance with this provision of the metal step, the TM mode which is a higher mode of the circular waveguide is generated to counterbalance a part of the electric field component of main mode TE so that the side lobes are reduced and so that the beam width in the E-plane is equal to the beam width in the H- plane. Another of said improvements includes the provision of radical pinnas, along the axis of the horn and, at the inner surface thereof or concentric circular pinnas provided at the inner surface of the horn perpendicularly to the axis of the horn. As the result of this provision of the pinnas, a part of the electric field in the E-plane is reduced. However, the optimum dimensions and the properposition of the metal step are difficult to ascertain and the frequency characteristic of the horn is necessarily affected since a section for adjusting the phase velocities of two modes so as to cancel out side lobes is necessary. On the other hand, the provision of pinnas is complicated, the weight thereof is large, and the resultant loss of electric energy is relatively high due to many channels between the pinnas.

An object of this invention is to provide a horn antenna wherein the field pattern has very low side lobes and identical beam widths with respect to the E-planc and the H-plane.

Another object of this invention is to provide a horn suitable for the feed radiator of a parabolic antenna.

Another object of this invention is to provide a horn capable of providing a parabolic antenna of low noise temperature.

Said objects and other objects of this invention can be attained by a horn antenna produced by flaring out the end of a circular or rectangular waveguide into the shape of a horn to be used for radiating or receiving radio waves directly into or from space or to be used as the feed radiator of an antenna for illuminating a mirror and/or a reflector. In accordance with a feature of this invention a circumferential band of dielectric material is arranged on a part of the inner surface of the horn so as to be symmetrical with the axis of the horn to obtain a field pattern having low side lobes and substantially the same beam width with respect to at least two planes of polarization perpendicular to each other. In accordance with another feature of this invention, the circumferential band of dielectric material may be replaced by two pairs of opposed plates of dielectric material arranged at longitudinally spaced positions from the aperture of the horn. In accordance a further feature of this invention, a cylindrical dielectric material may be further provided at the periphery of the aperture of the horn so as'to be symmetrical with the axis of the horn to obtain a field pattern having a flat top.

The principle of this invention will be better understood from the following more detailed discussion taken in conjunction with the accompanying drawings, in which the same or equivalent parts are designated by the same reference numerals, characters and symbols, and in which:

FIG. 1 shows characteristic curves explanatory of the principle of this invention;

FIG. 2 is a schematic perspective view for illustrating an embodiment of this invention;

FIG. 3 shows characteristic curves explanatory of the characteristic of the embodiment shown in FIG. 2;

FIG. 4 shows characteristic curves explanatory of the characteristic of a conventional horn antenna;

FIG. 5 is a schematic perspective view for illustrating another embodiment of this invention;

FIG. 6 shows characteristic curves explanatory of the characteristic of the embodiment shown in FIG. 5; and

FIGS. 7 and 8 are schematic perspective views each for illustrating another embodiment of this invention.

FIG. I shows a phase pattern, measured at 24 giga of a conical horn, having an aperture of 50 millimeters and a taper angle of 8 (eight degrees). In this phase pattern of FIG. 1, the abscissa is the rotation angle of the conical horn rotated with respect to the center of the aperture to measure the phase pattern, and the ordinate is the phase angle indicating the phase pattern where the angle of lead is positive. Moreover, the

characteristic pattern E is an E-plane phase pattern, and the characteristic pattern H is an H-plane phase pattern.

As understood from FIG. 1, the E-plane phase pattern E leads generally the H-plane phase pattern H,. The reason for this is the fact that, since the electric field pattern of E-plane is uniform while the electric field pattern of I-l-plane is sinusoidal in a case of excitation by the main mode TE. of the circular waveguide, the beam width in he E-plane is narrower than the beam width in the H-plane, so that the phase angle of lead for the E-plane is larger than the phase angle of lead for the H- plane. This principle is applicable to other horn antennas, such as horn antennas having rectangular sections, as well as the above-mentioned conical horn antenna.

In accordance with this invention, a dielectric layer is connected to the inner surface of a horn antenna so as to provide, for example a circular band. As a result of this construction, the phase of the electric field in the E-plane is delayed so that TM mode is equivalently generated at this point. Accordingly, if the dimension and position of the dielectric layer are suitably designed the side lobes are reduced by cancelling each other and, at the same time, the electric field pattern in the inside of the horn is symmetrical with the axis of the horn so that the radiation pattern of the antenna becomes symmetrical with the axis of the horn.

With reference to FIG. 2, an embodiment of this invention will now be described. This embodiment comprises a conical horn l, a circular waveguide 2 connected to the conical horn 1, and a circular band of dielectric material 3 (e.g.; polytetrafluoroethylene) seated at the inner surface of the conical horn I. If the, circular band of dielectric material 3 is eliminated, the electric field of main mode excited at the inside of waveguide 2 appears, through the taper of the horn I, at the as shown by reference numeral 4. However, since the circular band of dielectric material 3 acts so as to delay the phase of only the electric field component, this circular band of dielectric material 3 acts so as to delay the phase of the electric field at the upper or lower portion only of the illustrated conical horn, at which portion the field intensity is higher than at other portions. In other words, this circular band of dielectric material 3 has little effect on the left or right portion of the electric field 4 illustrated in FIG. 2. Accordingly, since the phase pattern with respect to the up-anddown direction (i.e.; E-plane) shown by an arrow A is delayed in comparison with the phase pattern of the H-plane, if the dimension, and position of the circular band of dielectric material 3 is designed so that the phase angle of lead determined by the dimension of the horn is cancelled out, the phase patterns of E-plane and of H-plane become identical with each other. Moreover, the centers of phases of each of the two planes become identical with each other, and cancellation of the side lobes and equalization of the widths of beams in E- and I-I-planes occur simultaneously in accordance with the effect of the TM mode generated therein. In this case, the formation of the circular band of dielectric material 3 is arranged along the inside surface and extends between two planes perpendicular to the axis of the horn I. The circular band of dielectric material of this formation acts uniformly to all polarized waves.

FIG. 3 shows measured results of the field pattern of the horn antenna illustrated in FIG. 2. It is understood from this field pattern that the side lobes are very low for the pattern E of E-plane and for the pattern I-l of H-plane, and that the beam width on the E-plane is substantially equal to the beam width on the H-plane. FIG. 4 shows measured results of the field pattern of a conventional antenna which is obtained by eliminating the circular band of dielectric material 3 from the horn antenna illustrated in FIG. 2. Effective merits will be clearly understood by comparing the results of the two patterns. It is well ascertained that the horn antenna of this invention has substantially the same characteristic as shown in FIG. 3 in the frequency range 21 giga-herz. Moreover, the input impedance characteristic (vswr) of the horn antenna of this invention is specified by a value lower than 1.02, so that this characteristic is not at all affected by the provision of the dielectric material 3.

With reference to FIG. 5, another embodiment of this invention will be described. In this embodiment, a cylindrical dielectric material 5 is further provided at the edge of the aperture of the embodiment shown in FIG. 2 so as to by symmetrical with respect to the axis of the horn I, FIG. 6 shows field patterns of this embodiment shown in FIG. 5. From the respective characteristic patterns 5,, and H, of E-plane and of H-plane, it is understood that while the side lobes are slightly higher thanthe characteristics shown in FIG. 3, the beam widths of the E-plane and I-I-plane are wider than the characteristic shown in FIG. 3 and substantially equal to each other. Moreover, the center portion of the beam becomes flat. These surprising effects are obtained from a flow of energy which is generated by reflection of energy of the peripheral part of the main beam caused by the newly provided dielectric material 5 and which travels to the center position of the horn I in the opposite phase to the main field. The flat beam is obtained by distribution of power of the center portion of the main beam. This horn antenna having such a flattened field pattern has high efiiciency for illumination in a case of the feed radiator of a Cassegrain antenna, since the field pattern is rectangularly shaped. Moreover, since the side lobes thereof are very low, loss of power leaked to the outside from the mirror is reducible. Accordingly, since a Cassegrain antenna having a large value of G/T can be readily realized in accordance with this invention, the horn antenna of this invention is suitable to antennas for satellite communication.

If the cylindrical dielectric material 5 only is used without the circular band of dielectric material 3 in the embodiment shown in FIG. 5, a field pattern having the flat portion is obtained but the beam width of the E-plane is quite different from the beam width of the I-I-plane and the side lobes become high. Accordingly, this formation cannot be used in practice.

The formation of the circular band of dielectric material 3 necessarily extends between two parallel planes perpendicular to the horn axis, but the edges of the circular band of dielectric material 3 may be irregularly cut out ifit acts uniformly to all polarized waves. Moreover, it is not essential that the circular band of dielectric material 3 is completely closed on the inner surface of the horn 1, although it must act uniformly to all of polarized waves.

The above-mentioned embodiments exhibit the effective merits in a relatively wide frequency range. However, in the case of a very wide frequency range those of embodiments the highest frequency exceeds by one and half times a lowest frequency b embodiment of the invention described below is more suitable.

In an embodiment ofthis invention shown in FIG. 7, a circular wave guide 2 and a conical horn l are similar to those of embodiments mentioned above. The curvilinear plates of dielectric material 3a and 3b are oppositely disposed at the upper and lower inner surface of the illustrated horn 1 so as to be symmetrical with the longitudinal axis 7 of the horn 1. Two other curvilinear plates of dielectric material 30 and 3a are 0ppositely disposed at the left and right inner surface of the illustrated horn 1 so as to be symmetrical with the axis 7 of the horn I, and spaced longitudinally from the plates 3a and 3b.

If a radio wave of main mode TE, having a frequency f and the main component of its electric field disposed in the vertical direction as shown by the arrow 8, is applied to the conical horn 1 through the circular waveguide 2, the travelling wave reaches the plates 3a and 30 without affection by the planes 3c and 3d, since the latter plates are arranged at the lower intensity positions for the electric field. In this case, since the plates 3a and 3b are arranged at positions of the highest intensity of the electric field, these plates 3a and 3b act effectively to the electric field so as to equivalently generate the TM mode by counter electric fields induced in the plates 3a and 3!). Accordingly, if the positions and dimensions of the two plates of dielectric material 3a and 3b are suitably designed, conditions for equal beam width" and low side lobes" can be satisfied for vertically polarized waves in a frequency range having the center frequency f,.

Next, if a radio wave of horizontally polarized wave having a frequency f, and polarized in the direction shown by an arrow 9 perpendicular to the arrow 8 is applied to the conical horn l, the TM, mode is generated similarly at the plates 3c and 3d. However, since the main direction of the electric field is in the direction of the arrow 9 and the plates 3a and 3b are arranged at the lowest positions of this electric field, the travelling waves are affected only by the plates 30 and 3d. Accordingly, if the positions and dimensions of the two plates of dielectric material 30 and 3d are suitably designed, conditions for equal beam width" and low side lobes" can be satisfied for horizontally polarized waves in a frequency range having the center frequency f It is ascertained that the length of the plates of dielectric material 30, 3b, 3c and 3d is sufficient in practice to be onethird to one-fifth the length of inner circumference at which the plates are arranged.

FIG. 8 shows another embodiment of this invention using a rectangular waveguide 2 and a quadrilateral pyramidal horn 1. Two pairs of plates of dielectric material (3a and 3b) and (3c and 3d) are respectively disposed at the upper and lower inner surfaces of the illustrated horn I and at the right and left inner surfaces of the horn 1. In this embodimennadvantages similar to those of the embodiment shown in FIG. 7'can be obtained. However, the main mode of the rectangular waveguide is the TE mode, and the higher mode corresponding to "IM mode of the circular waveguide is the TM mode. In this embodiment, the two plates 3a and 3b do not act on the horizontally polarized wave shown by an arrow 8, and the other two plates 3c and 3d do not act on the vertically polarized wave shown by an arrow 9. In other words, the two pairs of plates (3a and 3b) and (3c and 3d) have two active planes completely perpendicular to each other. Therefore, the design and adjustment of the antenna is easier than the embodiment shown in FIG. 7.

. '5 7 As mentioned above. the horn antenna of this invention has surprising merits while it has a simple construction. Moreover, horn antennas of high power can be provided in accordance with this invention. The embodiments shown in FIGS. 2 and 5 can be applied for linearly polarized waves, circularly polarized waves and elliptically polarized waves.

The continuous circumferential dielectric material 5 provided in the embodiment shown in FIG. 5 may be adapted to a pyramidal horn such as shown in FIG. 8.

Moreover, the horn antenna of this invention can be used as an independent antenna for radiating or receiving radio waves directly into or from space or as a feed radiator for illuminating a mirror or a reflector in a parobolic antenna, such as Cassegrain antenna.

What is claimed is:

I. An antenna for a waveguide comprising, an angularly flared horn antenna having a first end opening of major crosssectional area, and a second end of minor cross-sectional area for connection to a waveguide, and dielectric means attached to an inner surface of said horn antenna and including first and second portions of plate-shaped dielectric material positioned opp'osingly on said inner surface, each said portion extending circumferentially for at least one-fifth of the circumferential dimension of said horn antenna at said position of attachment to said horn antenna, and said portions being symmetrical with respect to a longitudinal axis of said horn antenna, whereby a first pair of field patterns of said horn antenna have low side lobes and substantially the same beam width with respect to at least two perpendicular planes of polarization.

2. An antenna for a waveguide as set forth in claim 1, in which said dielectric means comprises a first continuous band of dielectric material fixed circumferentially to said inner surface of said horn antenna at a position intermediate said first and second ends of said antenna.

3. An antenna for a waveguide as set forth in claim 2, further comprising a second continuous band of dielectric material fixed circumferentially to said inner surface of said horn antenna at said first end of said antenna, whereby the peaks of the field patterns are flattened.

4. An antenna for a waveguide as set forth in claim 1, in which said first and second portions of dielectric material comprise first and second separate strips of said dielectric material each having a pair of principal surfaces connected in contact with said inner surface of said horn antenna; and, in which said dielectric means further comprises third and fourth separate strips of dielectric material each having a pair of principal surfaces and having one of said principal surfaces connected in contact with said inner surface of said horn antenna; wherein said third and fourth strips are spaced from said first and second strips along said longitudinal axis of said antenna, are positioned symmetrically with respect to said longitudinal axis, and extend for at least one-fifth of the circumferential dimension of said horn antenna at said position of contact, and wherein a first straight line through a center portion of each of said third and fourth strips is longitudinally spaced from and disposed at an angle of with respect to a second straight line through a center portion of each of said first and second strips; whereby said first field patterns of said horn antenna are for a signal of frequency f and whereby a second pair of field patterns of said horn antenna for a signal of frequency having a main component of its electric field disposed at right angles to a main field component of said signal f have low side lobes, and substantially the same beam width with respect to at least two perpendicular planes of polarization of said signal f 5. An antenna for a waveguide as set forth in claim 4, further comprising a continuous band of dielectric material connected circumferentially to said inner surface of said horn antenna at said first end of said antenna, whereby the peaks of said first and second pairs of field patterns are flattened. 

1. An antenna for a waveguide comprising, an angularly flared horn antenna having a first end opening of major cross-sectional area, and a second end of minor cross-sectional area for connection to a waveguide, and dielectric means attached to an inner surface of said horn antenna and including first and second portions of plate-shaped dielectric material positioned opposingly on said inner surface, each said portion extending circumferentially for at least one-fifth of the circumferential dimension of said horn antenna at said position of attachment to said horn antenna, and said portions being symmetrical with respect to a longitudinal axis of said horn antenna, whereby a first pair of field patterns of said horn antenna have low side lobes and substantially the same beam width with respect to at least two perpendicular planes of polarization.
 2. An antenna for a waveguide as set forth in claim 1, in which said dielectric means comprises a first continuous band of dielectric material fixed circumferentially to said inner surface of said horn antenna at a position intermediate said first and second ends of said antenna.
 3. An antenna for a waveguide as set forth in claim 2, further comprising a second continuous band of dielectric material fixed circumferentially to said inner surface of said horn antenna at said first end of said antenna, whereby the peaks of the field patterns are flattened.
 4. An antenna for a waveguide as set forth in claim 1, in which said first and second portions of dielectric material comprise first and second separate strips of said dielectric material each having a pair of principal surfaces connected in contact with said inner surface of said horn antenna; and, in which said dielectric means further comprises third and fourth separate strips of dielectric material each having a pair of principal surfaces and having one of said principal surfaces connected in contact with said inner surface of said horn antenna; wherein said third and fourth strips are spaced from said first and second strips along said longitudinal axis of said antenna, are positioned symmetrically with respect to said longitudinal axis, and extend for at least one-fifth of the circumferential dimension of said horn antenna at said position of contact, and wherein a first straight line through a center portion of each of said third and fourth strips is longitudinally spaced from and disposed at an angle of 90* with respect to a second straight line through a center portion of each of said first and second strips; whereby said first field patterns of said horn antenna are for a signal of frequency f1, and whereby a second pair of field patterns of said horn antenna for a signal of frequency f2, having a main component of its electric field disposed at right angles to a main field component of said signal f1, have low side lobes, and substantially the same beam width with respect to at least two perpendicular planes of polarization of said signal f2.
 5. An antenna for a waveguide as set forth in claim 4, further comprising a continuous band of dielectric material connected circumferentially to said inner surface of said horn antenna at said first end of said antenna, whereby the peaks of said first and second pairs of field patterns are flattened. 